Large format sheet feed scanners may use contact image sensors (CIS) to create an image of the document being scanned. In a CIS scanner, pressure rollers are used to hold the document against the sensor. Because the CIS has a very narrow depth of field, any gap between the document and the top surface of the cover glass on the CIS may cause problems in the image of the scanned document. When the pressure rollers are not properly aligned, the pressure rollers may exert uneven amounts of pressure against the document as the document travels through the scanner.
For example, when one of the pressure rollers inside a CIS scanner is not perfectly cylindrical, an image quality (IQ) defect called “horizontal periodical banding” (HPB) may be produced in the image. This defect in the image is caused by the different pressure that the roller applies against the original document as it is being moved through the scanner. Since the roller is not perfectly cylindrical, but the rotational axis of the roller is fixed, the effective radius of the pressure roller varies (this condition is sometimes called pressure roller eccentricity), and so does the pressure applied against the scanned paper. This variation in pressure may cause the distance between the scanned paper and the glass plate on the top of the sensor to vary. The variation in distance causes small differences in the focus of the image and may cause small shifts in the color and/or intensity of the image.
In another example, the roller may be cylindrical, but the cylindrical surface of the roller may not be aligned with the axis of rotation of the roller. This will also cause a variation in the effective radius of the roller and cause a periodic variation in the pressure applied by the roller. In some cases the pressure applied by the roller may vary enough to cause the document to become separated from the top surface of the sensor. The change in distance between the document and the sensor may cause a periodic change is focus and a periodic shift in the color and/or the intensity of the image.